The invention is directed to solar cells, such as multi-junction solar cells made by wafer bonding and layer transfer.
There is considerable interest in the design and fabrication of tandem multi-junction solar cells for high efficiency photovoltaics for space-based and terrestrial applications. Multi-junction solar cells consist of two or more p-n junction subcells with band gaps engineered to enable efficient collection of the broad solar spectrum. The subcell band gaps are controlled such that as the incident solar spectrum passes down through the multi-junction solar cell it passes through subcells of sequentially decreasing band gap energy. Thus, the efficiency losses associated with single-junction cells, i.e., the inefficient collection of high-energy photons and failure to collect low-energy photons, are minimized. Multi-junction solar cells are generally fabricated using a monolithic process that leads to series connected subcells. Electrical connection between subcells is performed by a heavily doped tunnel junction formed during the growth of the multi-junction structure. In the devices described below, it is understood that the subcells are electrically connected with such a structure. In series-wired multi-junction solar cells, it is important for the photo-current generated by solar radiation in each subcell to be closely matched to all other subcells in the multi-junction solar cell structure. This design constraint along with materials integration limitations due to crystal lattice-matching requirements that accompany the use of epitaxy to fabricate multi-junction solar cells limit the number of subcells that can presently be integrated.